Recent liquid crystal displays have features such as low power consumption, low operational voltages, light weight, and reduced thicknesses, and thus are used as various display devices. Liquid crystal displays are made up of many elements such as liquid crystal cells, a polarizing plate, a retardation film, a light collecting sheet, a diffusion film, a light guide plate, and a reflection sheet. Therefore, many efforts have been made to achieve objects such as productivity improvements, weight reductions, and brightness improvements by reducing the number or thicknesses of films or sheets.
Current polarizing plates have a stacked structure in which a protective film or protective films are attached to one or both surfaces of a polarizer formed of a polyvinyl alcohol (PVA)-based resin generally dyed with a dichroic dye or iodine, by an adhesive. In the related art, triacetyl cellulose (TAC)-based films have mainly been used as protective films for polarizing plates. However, TAC films are easily deformed in high-temperature, high-humidity environments. Therefore, protective films made of various materials have recently been developed to replace TAC films. For example, methods of using films such as polyethylene terephthalate (PET) films, cycloolefin polymer (COP) films, and acrylic films individually or in combination have been suggested.
Aqueous adhesives usually formed of PVA-based resin solutions have been used as adhesives for attaching protective films to polarizers.
However, if aqueous adhesives are used for protective films other than TAC films, such as acrylic films or COP films, the adhesiveness of the aqueous adhesives becomes poor. Therefore, the use of aqueous adhesives is limited to kinds of film materials. In addition to the poor adhesiveness of aqueous adhesives to some materials, if protective films formed of different materials are attached to both surfaces of a polarizer by using an aqueous adhesive, curling may appear on a polarizing plate after a drying process of the aqueous adhesive, and initial optical properties of the polarizing plate may be worsened. Furthermore, since aqueous adhesives inevitably require a drying process, the defect rate may be increased because of differences in water permeability and thermal expansion rates during the drying process. To address these problems, methods of using nonaqueous adhesives instead of aqueous adhesives have been proposed.
Nonaqueous adhesives that have been proposed as adhesives for polarizing plates may be classified into a radically curable type and a cationically curable type according to curing methods. Although cationically curable adhesives have high degrees of adhesiveness with respect to various films, the curing rate and curing degree thereof are low, and thus it is disadvantageous to use cationically curable adhesives in a manufacturing process. To overcome theses demerits of cationically curable adhesives, radically curable adhesives including an acrylamide-based compound as a main ingredient have been proposed. Radically curable adhesives including an acrylamide-based compound as main an ingredient can be cured more rapidly than cationically curable adhesives. However, the curing rate and adhesiveness of radically curable adhesives having an acrylamide-based compound as a main ingredient are lowered in high-humidity environments. In addition, since polarizing plate manufacturing processes include a wet process in which PVA films are swelled, dyed, and stretched in an aqueous solution, polarizers contain a large amount of moisture, and thus an additional pretreatment preprocess for drying polarizers with hot air or treating surfaces of polarizers with plasma is necessary for manufacturing polarizing plates using such acrylamide-based radically curable adhesive composition adhesives.
Therefore, there is a need for a radically curable adhesive maintaining a curing rate and adhesiveness thereof, even in high-humidity environments and thus which is applicable to polarizing plates without having to perform an additional process.